Characteristics of Shale Reservoir and Sweet Spot Identification of the Lower Cambrian Niutitang Formation in Northwestern Hunan Province, China

Abstract

The accumulation and productivity of shale gas are mainly controlled by the characteristics of shale reservoirs; study of these characteristics forms the basis for the shale gas exploitation of the Lower Cambrian Niutitang Formation (Fm), Southern China. In this study, core observation and lithology study were conducted along with X‐ray diffraction (XRD) and electronic scanning microscopy (SEM) examinations and liquid nitrogen (N2) adsorption/desorption and CH4 isothermal adsorption experiments for several exploration wells in northwestern Hunan Province, China. The results show that one or two intervals with high‐quality source rocks (TOC>2 wt%) were deposited in the deep‐shelf facies. The source rocks, which were mainly composed of carbonaceous shales and siliceous shales, had high quartz contents (>40 wt%) and low clay mineral (<30 wt%, mainly illites) and carbonate mineral (<20 wt%) contents. The SEM observations and liquid nitrogen (N2) adsorption/desorption experiments showed that the shale is tight, and nanoscale pores and microscale fractures are well developed. BJH volume (VBJH) of shale ranged from 2.144×10–3 to 20.07×10–3 cm3/g, with an average of 11.752×10–3 cm3/g. Pores mainly consisted of opened and interconnected mesopores (2–50 nm in diameter) or macropores (>50 nm in diameter). The shale reservoir has strong adsorption capacity for CH4. The Langmuir volume (VL) varied from 1.63 to 7.39 cm3/g, with an average of 3.95 cm3/g. The characteristics of shale reservoir are controlled by several factors: (1) A deep muddy continental shelf is the most favorable environment for the development of shale reservoirs, which is controlled by the development of basic materials. (2) The storage capacity of the shale reservoir is positively related to the TOC contents and plastic minerals and negatively related to cement minerals. (3) High maturity or overmaturity leads to the growth of organic pores and microfractures, thereby improving the reservoir storage capacity. It can be deduced that the high percentage of residual gas in Niutitang Fm results from the strong reservoir storage capacity of adsorbed gas. Two layers of sweet spots with strong storage capacity of free gas, and they are characterized by the relatively high TOC contents ranging from 4 wt% to 8 wt%.

The accumulation model of organic matters for the Niutitang Formation shale and its control on the pore structure: a case study from Northern Guizhou

“…Once the R o exceeds 3%, the organic pores in the shale nearly disappear (Wang et al 2016a, b, c;Gu et al 2018;Wang et al 2020a, b). The content of quartz is high in the Niutitang Formation shale (Qin et al 2019), and the quartz presents with five different types, silt-size/sand-size detrital quartz, siliceous skeletons, overgrowth nucleated around detrital quartz, matrix-dispersed microcrystalline quartz and aggregates of euhedral quartz, respectively, and the authigenic quartz is the dominant type (Dong et al 2021). However, Liu et al (2018) reported that the high content of quartz in the Niutitang Formation shale profits from the biogenetic quartz (Dong et al 2021).…”

The Characteristics and Main Controlling Factors for the Formation of Micropores in Shale from the Niutitang Formation, Wenshuicun Section, Southwest China

“…More and more researchers are paying attention to the development of these pores and their controlled factors, and have discovered many important results. Generally speaking, shale pore development is mainly affected by thermal maturity, organic matter abundance, kerogen maceral and mineral content, as well as other factors [10][11][12][13][14][15]. The relationship between thermal evolution degree and pore structure characteristics has obvious stage characteristics, which means that the porosity increases when the Ro(vitrinite reflectance) of the Niutitang shale reservoir is between 0.6% and 3%, and the pore structure is well developed; however, when Ro is more than 3.5%, it is adverse to pore development, and can even destroy the original pore structure and eventually lead to the disappearance of the pores [10,16].…”

“…The relationship between thermal evolution degree and pore structure characteristics has obvious stage characteristics, which means that the porosity increases when the Ro(vitrinite reflectance) of the Niutitang shale reservoir is between 0.6% and 3%, and the pore structure is well developed; however, when Ro is more than 3.5%, it is adverse to pore development, and can even destroy the original pore structure and eventually lead to the disappearance of the pores [10,16]. Some scholars have also proposed that organic pores and micro-cracks develop in large quantities in the high-mature to over-mature stage [11,12]. In the polar development stage of organic pores, there is a positive correlation between TOC and pore volume of different pore sizes and total pore volume [13,14].…”

Effect of Thermal Maturity on Pore Type and Size in Transitional Shale Reservoirs: An Example from the Upper Paleozoic Shanxi Formation, Ordos Basin, China

“…Mesopores are commonly acknowledged to dominate the pore volume, and mineral matrix pores can also be observed in transitional shale, but organic matter (OM) pores are uncommon. 5−13 Moreover, the pore volume in marine shale is relatively equal to that in transitional shale according to the experimental analysis of gas adsorption, 6,14,15 which indicates the potential of transitional shale gas production and is worth further study.…”